A variety of refrigerant compressors for use in vehicle air conditioning systems are currently available. A popular vehicle compressor design is the variable displacement axial type. In this type of compressor, a number of cylinders are equally angularly spaced about and equally radially spaced from the axis of a central drive shaft. A piston is mounted for reciprocal sliding motion in each of the cylinders. Each piston is connected to a swash plate or wobble plate received about and operatively connected to the drive shaft. During operation of the compressor, rotation of the drive shaft imparts a wave-like reciprocating motion to the wobble plate. This driving of the wobble plate in a nutating path about the drive shaft serves to impart a linear reciprocating motion to the pistons. By varying the angle of the wobble plate relative to the drive shaft, the stoke of the pistons and, therefore, the displacement or capacity of the compressor may be varied to effect the desired compressing action.
A shortcoming realized in this type of compressor system is the degraded performance resulting from gas pressure pulsations discharged from the compressor. While the piston type compressor provides an effective way to compress and circulate the refrigerant fluid throughout the system, it has the adverse side effect of delivering high pressure pulsations coincident with the discharge stroke of the pistons. In addition to creating a noisier and rougher operating system, these discharge pressure pulsations also lead to premature fatigue and failure of component parts throughout the air conditioning system, thereby diminishing its reliability.
Various attempts have been made to reduce these pressure pulsations in order to provide smoother and quieter running systems. One accepted approach is to provide a restriction, such as a restricted nozzle, in the discharge port of the compressor. As another example, U.S. Pat. No. 4,652,217, issued Mar. 24, 1987, to Shibuya shows a compressor with an attenuation device in the form of a porous plug in the valve plate (see FIGS. 4 & 5). This attenuation device operates in the same manner as a standard restricted orifice by limiting the rate that gas is permitted to escape the discharge chamber. This discharge rate is regulated by the size and number of the restriction holes provided in the porous plug. The system is configured such that, during the piston discharge stroke, a much greater volume of fluid is discharged from the cylinder bore into an intermediate discharge cavity than the restriction holes allow to pass through the valve plate to the final discharge chamber. This results in a variable pressurized region within the intermediate discharge cavity. This pressure buildup is evidenced during the piston intake stroke by a continual, and thus somewhat smoothed flow of pressurized gas to, and consequently from the discharge chamber to the outlet port. In this way, the net effect of the restriction is to stretch the compressor discharge over the entire operational cycle (both intake and discharge strokes), rather than just during the discharge stroke. The restriction also provides some attenuation by interaction of the established stream pulsations with incident pressure waves adjacent the restriction. This action works by cancellation of opposing fluid forces, as is well known.
A different type of attenuation assembly is shown in U.S. Pat. No. 4,274,813 to Kishi et al., issued Jun. 23, 1981, in which a plurality of baffles or dividers are provided in cascade fashion within the discharge chamber. These baffles have a similar effect on the pressure pulsations resulting from the discharge stroke of the piston, as did the restriction holes in the Shibuya patent. That is, they provide a restriction so as to prevent the fluid pulses from freely propagating through the discharge chamber to the outlet port of the compressor. The result is a similar buildup of pressure within the discharge chamber during the piston discharge stroke. The increased pressure, like in the Shibuya patent, is bled away during the piston intake stroke, thereby spreading the discharge over the entire operational cycle. Also, to some extent the pulsations and incident pressure waves cancel each other out, as in the other arrangements.
While these design approaches have realized performance and reliability improvements over other attempts, they enjoy only limited success. One reason for this limitation is due to the constant or static orifice area provided for the fluid discharge by the restriction hole(s) or baffles.
More specifically, and as elementary laws of fluid dynamics confirm, the volumetric rate at which fluid is transferred in a compressor from the discharge chamber through the outlet port and into the outgoing refrigerant line is directly proportional to the pressure differential between the discharge chamber and the refrigerant line, as well as the size of the orifice area connecting the two. Therefore, in order to maintain a substantially constant pressure at the output port and within the refrigerant line, and thus maximize attenuation of pulses, the area of the connecting orifice should be variable and controlled according to the changing pressure within the discharge chamber. It is readily observed that any design approach providing only a static or constant area orifice still exhibits notable pressure pulsations at the output. Accordingly, a need clearly exists for a design with a variable orifice to further reduce the gas pressure pulsations from a compressor.